1. Field of the Invention
The present invention relates to an aromatic polycarbonate, an electrophotographic photoreceptor, and an image forming apparatus.
2. Description of the Related Art
Heretofore, in an electrophotographic image forming apparatus such as copying machines, printers, and facsimile machines, polycarbonate resins have been used as a surface layer of an electrophotographic photoreceptor forming electrostatic latent images since they have high mechanical strength and are excellent in wear resistance, and dimensional stability. Particularly, in a function separation type electrophotographic photoreceptor formed by successively laminating, on a conductive substrate, a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance, polycarbonates have often been used for the charge transporting layer as an outermost layer as a binder resin for retaining the charge transporting substance in the layer.
As the polycarbonate, aromatic polycarbonates such as bisphenol A polycarbonate obtained by reacting 4,4′-(1-methylethylidene)bisphenol (bisphenol A) and phosgene, and a Z type polycarbonate synthesized from 4,4′-cyclohexylidene bisphenol (bisphenol Z) excellent in wear resistance and solubility have been known as typical examples. Such polycarbonates are generally excellent in the mechanical strength, particularly, mechanical strength at low temperature and also excellent in weather resistance compared with other thermoplastic resins.
By the way, for maintaining electric properties required for the electrographic photoreceptor, the charge transporting layer contains a charge transporting substance in an amount substantially identical with that of the polycarbonate as the binder resin. However, such a ratio of the amount between the polycarbonate and the charge transporting substance causes deterioration of the wear resistance and the durability of the charge transporting layer also along with the fact that the charge transporting substance is low molecular weight compound having a less compatibility with the polycarbonate.
Accordingly, it has been attempted to decrease the addition amount of the charge transporting substance and improve the durability of the charge transporting layer while maintaining the electrical property as the electrophotographic photoreceptor by improving the electric property of the charge transporting substance. A further improvement for the sensitivity and the mobility of the charge transporting substance has been intended by the attempt. However, for the charge transporting substance proposed so far, an addition amount to such an extent as causing an undesired effect on the durability of the charge transporting layer is necessary in order to obtain a desired electric property. Therefore, it is extremely difficult to make the electrical property and the durability compatible.
For solving the problem in the electrophotographic photoreceptor described above, it has been attempted to provide the binder resin with a charge transport capability thereby decreasing the addition amount of the charge transporting substance, and development has been proceeded for a binder resin containing a constituent unit having the charge transport capability, that is, a so-called polymeric photoconductive material. Specific examples thereof include carbazole type polycarbonates having a carbazole ring in the main chain (refer, for example, to Japanese Unexamined Patent Publication JP-A 4-183719 (1992)) and polyvinyl carbazoles having a carbazole ring on the side chain (refer, for example, to Japanese Unexamined Patent Publication JP-A 50-82056 (1975)). The carbazole type polymers involve drawbacks that they are poor in the flexibility and, also have low mechanical strength and, further, they tend to form structural traps as one of the causes for deteriorating the charge transport capability.
Further, there are polystyrene compounds having hydrazone side chains (refer, for example, to Japanese Unexamined Patent Publication JP-A 3-50555 (1991)), aryl amine compounds having a tertiary amine structure in the main chain (refer, for example, to Japanese Unexamined Patent Publications JP-A 1-13061 (1989), JP-A 1-19049 (1989) and JP-A 5-40350 (1993)), and (meth)acrylic acid copolymers having a tertiary amine structure (triphenylamine skeleton) on the side chains (refer, for example, to Japanese Unexamined Patent Publication JP-A 5-66598 (1993)). Also the polymers are not sufficiently satisfactory in view of the charge transport capability and the mechanical strength.
Further, although all of the polymers described in JP-A 4-183719, JP-A 50-82056, JP-A 3-50555, JP-A 1-13061, JP-A 1-19049, JP-A 5-40350, and JP-A 5-66598 is a π-electron conjugation system, there are polysilanes having a conduction mechanism by way of a σ-electron conjugation system different therefrom (refer, for example, to Japanese Unexamined Patent Publications JP-A 63-285552 (1988) and JP-A 5-19497 (1993)). While polysilanes are excellent in the charge transport capability, since they are easily decomposed by an ultraviolet light and also poor in the mechanical strength, they have not yet been to practical use.
Further, there are polycarbonate type copolymers prepared by using a monomer for the charge transporting substance and copolymerizing the monomer with a polycarbonate monomer with an aim of improving the mechanical strength (refer, for example, to Japanese Unexamined Patent Publications JP-A 3-221522 (1991) and JP-A 4-11627 (1992)). However, such copolymers are not at a sufficiently satisfactory level both in the charge transport capability and the mechanical strength.
Further, there is an aromatic polycarbonate prepared by using a monomer as an enamine compound excellent in the charge transport capability (refer, for example, to Japanese Unexamined Patent Publication JP-A 6-43674 (1994) and the specification of Japanese Examined Patent Publication JP-B2 3580426) and copolymerizing the enamine monomer with a polycarbonate monomer (refer, for example, to Japanese Patent Unexamined Patent Publication JP-A 2004-269813). However, since the aromatic polycarbonate has a high structural symmetry tending to cause crystallization, this results in a disadvantage that crystallized portions form image defects when it is used as a resin for a photosensitive layer of an electrophotographic photoreceptor.
Further, it has also been conducted to laminate a surface protection layer on a photosensitive layer in order to improve the mechanical strength of the photosensitive layer such as a charge transporting layer. The surface protection layer usually contains a conductive material and a binder resin. Then, for the binder resin it has been proposed, for example, polycarbonates having polar groups (refer, for example, to U.S. Pat. No. 4,260,671 A) and polycarbonates having fluoro substituted alkyl groups (refer, for example, to Japanese Unexamined Patent Publication JP-A 10-158380 (1998)). However, in such electrophotographic photoreceptor, since a potential barrier is formed at the boundary between the surface protection layer and the charge transporting layer due to the disconformity between the charge transporting substance in the charge transporting layer and the conductive material in the surface protection layer, injection of charges is insufficient to lower the sensitivity and at least one of peeling or cracking occurs during manufacture due to the difference of the shrinkage caused by the difference of materials in both of the layers failing to attain a sufficient performance.
Further, a surface protection layer comprising a siloxane type resin has been proposed recently (refer, for example, to Japanese Unexamined Patent Publication JP-A 2000-242019). However, heat curing of the siloxane type resin is necessary for forming the surface protection layer by using the siloxane resin, but the material in the photosensitive layer may sometimes be degraded by heating. Further, since the siloxane resin per se is an insulator, it is necessary to add a low molecular compound having the charge transport capability but addition of such a low molecular compound also causes lowering of the mechanical strength of the surface protection layer like in the charge transporting layer.